A mouse model has been developed in which an immunological response triggered by the induction of a graft-versus-host reaction (GVHR) results in the development of lymphoreticular tumors. There is evident that the GVHR activates a latent or unexpressed murine leukemia virus (MuLV) and that the activated virus is responsible for tumor formation. A possible mechanism of viral activation is that lymphocytes stimulated during the GVHR release and/or replicate MuLV. We propose: 1) to study replication of MuLV added to normal lymphocytes stimulated in vitro by either mitogens, specific antigen, or the mixed lymphocyte reactions; 2) to study release of endogenous MuLV by normal lymphocytes exposed in vitro to various inductive stimuli, such as somatic hybridization and 5-iododeoxyuridine; 3) to study release of endogenous MuLV and/or replication of added MuLV in lymphocytes from GVHR-animals; 4) to study the ability of MuLV isolated from GVHR-animals to replicate in cells other than dividing lymphocytes. MuLV will be assayed by the XC test and the reverse transcriptase assay.